I am in the final stages of purchasing an X728. The other finalist was the possible choice of an X748 (diesel). Originally, the diesel was my choice, however in 2011 models (due to consessions made for California) JD significantly reduced the diesel's engine size (although HP is same) and torque has dropped accordingly (from 45.7 to 40.6).
The X728 is cheaper and has more HP than the X748 (27 vs24). I've not found specs to compare torque at RPM, but they probably have grown closer.
Since I will use the machine mostly for mowing, and running higher RPMs, it seems to me that HP gets the nod over torque, especially considering the price diferential.